2.0 Analysis 2.1 Introduction The analysis will address the effect of a loss of tail rotor authority while conducting longline operations within the shaded area of the H-V curve, and the reason why the input bevel gear of the 42-degree gearbox failed. 2.2 External Load Operations versus H-V Curve Although the helicopter was operated at a low height and low speed combination within the shaded area of the H-V curve, the external load operation was conducted in accordance with the approved External Load Operation Supplement to the basic flight manual. Longline, external load operations such as drill support require that helicopters be operated routinely at height and speed combinations within the shaded area of the manufacturer's H-V curve. Although such missions can be carried out in accordance with the external load supplement to the basic flight manual, operation within the shaded area of the H-V curve is nonetheless risky as a safe landing cannot always be accomplished should a critical emergency such as an engine or tail rotor failure occur. Helicopter pilots who specialize in longline, external load operations routinely fly within the shaded area of the manufacturer's H-V curve, and are aware that such an operation is of a high-risk nature. However, during external load operations within the shaded area of the H/V curve, pilots should be prepared not only for a possible loss of power, but also for an unexpected loss of tail rotor authority, which is more likely to occur under high loads, such as during hover at maximum weights. 2.3 Release of External Cargo When the input bevel gear failed, there was little time for the pilot to recognize that he had lost tail rotor authority. In this situation, above the trees, the pilot's response was to immediately attempt to release the external load from the external cargo hook and prepare for a crash landing. His intention was to arm the cargo release mechanism by pulling outward on the cargo release arm switch on the collective control column with his left thumb and wiping it downward, in accordance with the way the switch had been reinstalled. The pilot also attempted to kick the manual-release pedal located between the rudder pedals. However, in the presence of the gyrations of the helicopter following the loss of tail rotor authority, the pilot did not succeed in releasing the external cargo. 2.4 Input Bevel Gear Failure Failure analysis of the input bevel gear of the 42-degree gearbox determined that the gear had fractured and failed due to relatively high-cycle, low-stress fatigue which happened during flight at a root fillet on the concave face or drive side of a tooth. The failure of the input bevel gear occurred at a critical phase of flight, as the helicopter was flying over the trees at a low height and low speed combination which precluded the pilot from completing a safe autorotative landing. 2.5 Use of Available Safety Equipment The pilot was not wearing his helmet, and was not using the shoulder harness. It was not determined to what extent this contributed to the severity of the pilot's injuries. 3.0 Conclusions 3.1 Findings The external load operation was being conducted in accordance with the approved External Load Operation Supplement to the basic flight manual. The input bevel drive gear failed due to a high-cycle, low-stress fatigue mode of progressive cracking. The pilot lost tail rotor authority because the input bevel gear of the 42-degree gearbox fractured, which disconnected the drive shaft power from the main transmission to the tail rotor. With existing gyrations of the helicopter following the loss of tail rotor authority, the pilot did not succeed in releasing the external cargo. The loss of tail rotor authority occurred while the helicopter was operating over a forested area at a low height and low speed combination which precluded the pilot from performing a safe autorotative landing. The pilot was not wearing his helmet, and was not using the available shoulder harness. 3.2 Causes The pilot lost tail rotor authority because the input bevel gear of the 42-degree intermediate gearbox fractured due to a high-cycle, low-stress fatigue mode of progressive cracking, disconnecting the drive shaft power from the main transmission to the tail rotor. 4.0 Safety Action 4.1 Action Taken Following receipt of a TSB Safety Information Letter concerning previous 42-degree gearbox failures, Transport Canada (TC) published an article in the 1/93 issue of Maintainer, advising operators involved in repeated heavy-lift operations of the history and risk of 42-degree gearbox failures. Additionally, TC issued a letter, dated 06 October 1993, to owners of Bell 204B and 205A-1 helicopters. The letter advises that the 42-degree gearbox on the referenced models of helicopter is prone to failure on helicopters used in repeated, external heavy-load lifting, even if the maximum load carried is within limits. Transport Canada Aviation is monitoring the situation, and has been in contact with the Federal Aviation Administration and Bell Helicopter Textron Inc. regarding corrective action for the existing problem. Bell Helicopter Textron Inc. has issued Operations Safety Letters OSN-205-93-31 and OSN-GEN-93-25, which caution operators about heavy lift operations.